Process of obtaining alumina



lTED STATES RALP H S. SEER-WIN, OF EAST ST. LOUIS, ILLINOIS, ASSIGNOR TOALUMINUM COM- PANY OF AMERICA, OF PITTSBURGH, PENNSYLVANIA, ACORPORATION OF PENN- SYLVANIA.

rnocnss or OBTAINING ALUMINA.

No Drawing. Application filed January 16,

Z '0 (IN whomit may concern Be it known that I. RALPH S. Snnmvxx,-rcs1dent of East St. Louis. in the county of St. (lair and State ofIllinois. have invented .ane and useful Improvement in Processes ofObtaining Alumina. of which the followmg is a specification.

This invention relates to the so-called fulnacing processes ofseparating alumina from aluminous material, that is, processes in whichthe aluminous material, with a suitable reagent is subjected to asintering temperature. the sintered material afterward crushed andleached. and the alumina precipitated from the lye or solution soformed.

The purpose of the present invention is to adapt the furnacing processto the treat-- ment of aluminous material containing such a proportionof silica. either in chemical combination or in mechanical mixturetherewith. as to render it uneconomical .to treat according to the usualcommercial processes. Among the aluminous materials to which the processis particularly adapted are low grade bauxites. which contain aconsiderable percentage of silica mostly in mechanical mixture. naturalclays. principally silicates of alumina, and the so-called red mud whichis the solid residue resulting from the wet or digester treatment ofbauxite and similar aluminous earths for the separation of aluminatherefrom, and

which contains a large amount of silica,

much of it in chemical combination with the alumina. and whichheretofore has been con sidered merely as waste material useful forfilling for low land or similar uses.

The large. amount of silica contained in the low grade bauxites. clay.and the socalled red mud." and especially the silica which is inchemical combination with the alumina. has rendered the usual commercialprocesses for separating alumina from high grade bauxites impractical.or at least un economical to apply to said low grade materials. Thesilica in these lower grade and waste materials holds the alumina andprevents it from being dissolved by the ordinary processes of recoveringalumina.

The process. generally stat-ed. consists in Specification of LettersPatent.

Patented July 1, 1922.

1914, Serial No. 812,594. Renewed April 3, 1919. Serial half of theusual amount) subjecting the recovering the requisite amount of sodanecessary to be applied in the furnace from the residual liquor andusing the balance of the residual liquor over again as the leachmgagent.

Inasmuch as the silica contained in the low grade aluminous materialsreferred to. isthe cause of the insolubility of the alumina. it isnecessary to bring a particle of the alkaline material. such forinstance, as lime. into contactwith practically each particle of silica.to form a silicate, for instance. calcium silicate. It is thereforenecessary that the reagents be very intimately mixed with the aluminousmaterial, and as the materials must in any event be reduced to a finecondition, preferably a pulverulent condition, it is preferred to securethis intimate mixture by grinding the two materials together.thealuminous material having first been dried.

The preferred alkaline reagent is lime. with the addition of a smallamount of sodium compound. but any form of alkaline material may beused. The lime when used may be either in the form of burnt lime ao), orlimestone (Ca((),),orslaked lime ((a(()H) The sodium compound preferablyis soda ash, but may be sodium carbonate, caustic soda, sodium chloride,or any other convenient cheap form of sodium com pound.

The exact amount of lime and of sodium compound to be added will varywith the composition of the aluminous material being treated. Themolecular. proportions'of the lime to the silica (a0 SiO may vary from2.8 to 1, to 1.2 to 1. and the molecular proportions of the sodiumcompound to the alumina (Na O:Al O,) may vary from 2 to are sodiumcompound equivalent to Linolecule of Na,() to 1 molecule of alumina and2 molecules of lime to 1 molecule of silica.

This mixture is heated to a sintering temperature in any suitable way,preferably in V i a rotary kiln, and for such length of time as to bringabout the reactions whereby there is formed insoluble silicate of limeand sodium aluminate in soluble form, The tem perature must be soregulated that the material is fritted together but is not melted to aglass, as it is diflicult to leach out the material if it is fused. Thesintered material so formed is then broken up (and preferably ground)and leached, forming a sodium aluminate solution or lye, and leaving aresidue which is principally calcium silicatc, with such otherimpurities as oxids of iron, titanium or the like, as are contained inthe low grade bauxite, clayor red mud being worked.

The e act reactions taking place in the furnace @a matteron whichopinions differ, but the essential thing is to get rid of the largeamount of silica, whether in mere mixture or in chemical combinationwith the alumina, by causin it to combine with the lime and form a cacium silicate, and thereby free thealumina and sodium compound. Itisalso essential that the alumina be held in a form in which it issoluble, and therefore recoverable by leaching, preferably by combiningit with the sodium compound to form sodium aluminate. The final resultaimed at, therefore, is to get the silica into combination with analkaline earth material, forming insoluble silicates, such as thesilicate of calcium, and have the alumina held in soluble form,preferably as sodium alumina'te, and it is quite immaterial whether 'thereactions that take place form a single silicateor two or threedifferent silicates.

The sintered material (preferably finely ground) 'is then leached withsodium compound liquor, preferably a solution of caustic soda, such, forinstance, as the sodium aluminate solution from which all aluminaavailable for recipitation has been previously preci'pltated, that is,the residual soda liquor resulting from the ordinary methods ofprecipitating alumina (e. g., the auto-precipitation process, such asthe Bayer process and its modifications). By using a soda leachingsolution tlga amount of sodium compound which need be added in thefurnace, may be small, practically one-half of what would otherwise berequired, the balance of sodium compound being supplied during leaching.It furthermore makes it necessary to evaporate to dryness only a portionof the soda liquor resulting from the auto-precipitation process, orsuch other precipitation process as may beemployed, that is, onlysufficient to recover the/necessary amount of sodium compound which mustbe added to the furnace, the remainderof the liquor being used as theleaching liquor, thereby effecting a very considerable economy in theevaporation of the soda liquor.

The soda solution used for leaching out product combined '(i, e.,

NaOl-lto give from 4 to 5 molecules of NaOH in the leaching liquorandsintered product combined for each molecule of Al,(),, in the leaching1i uor and sintered l a OzAl Q, ratio is between 2:1 and 2.521). It theabove mentioned residual sodium aluminate, solution available forleaching does not contain this quantity of soda, additional soda must beadded to make up the deficiency.

In leaching the sintered product, the leaching solution is preferablyheated above 180 F. in order to render the material readily soluble andshorten the period of time required for leaching. Care, however, must betaken not to heat the leaching solution to too high a temperature, as atemperature at or above the boiling pointof water (212 de- .re-nder theleaching procem rapid, but is not sol igh as to render the sodium andalumina insoluble. Leaching at a temperature between 180-and 212 F. is avery important point in the process and has not been previously noted byany other investigator.

In the liquor or lye thus produced by leaching the sintei'd material thealumina is held in solution by the caustic soda. and

the solution also contains silica in quantities which may be as much as2 per cent of the alumina. The solution must be purified ta remove thisdissolved silica therefrom and this is preferably done by heating thesolution in a closed vessel for about two hours to a temperature ofabout 290 degrees Fahrenheit, corresponding to a pressure of about 40 or50 pounds per square inch above atmospheric. It is notcnecessary tolimit the operation to this exact'tempcrature or to this exact period ofdigestion, as they may be changed to suit circumstances; but in generalthe higher the temperature the shorter the period of time Wequired fordigestion.

. It is not necessary, in all cases, to 'Jrec-ipitate all the silicafrom. the solution by this means, for the reason that when the liquor isafterwards treated for the precipitation of the alumina therefrom (bythe Bayer or Fickes process) a certain amount of silica remains in thesolution even after the available aluminahas been precipitated out. Butif the alumina in the aluminate liquor result ing from the leaching isto be precipitated out by the (0. method, it is necessary to digest thisleaching liquor at a high temperature. for several hours in order toremove practically all the si ica.

The silica and other materials which are precipitated by this digestertreatment are filtered off, leaving a sodium aluminate solution fromwhich the alumina may be recovered by any of the methods now in use forprecipitating alumina from such as by passing carlmnicacid gastherethrough, or by agitating the solution, or by the so-called Bayerprocesswhich consists in adding to or inoculating the sodium aluminatesolution with freshly precipitated aluminum hydrate, and then agitatingfor a long time (about three days), or by any other known. method ofprecipitating alumina from such solutions. 'hichever process is employedresults in precipitating thealumina, which is filtered off and washed,while the solution remaining is in part evaporated to dryness to recoversufficient sodium compound for the next furnacing operation, and theremainder used over again for leachng the crushed sintered mass. Thisresidual liquor is a solution of caustic 0(13; and sodium aluminate ot'comparatively low alumina content, and is especially useful the leachingliquor.

An especially valuable feature of the foregoing process is the reductionin the amount of the sodium compound added to the other ingredients ofthe furnace charge and supplying the balance of the sodium compound inthe leaching liquor, which preferably is the sodium aluminate liquorremaining after the precipitation of the alumina from a sodium aluminateliquor. Both thesoda solution for leaching, as well as the sodiumcompound which is added to the furnace, are practically allrecoveredfrom the residual liquor after the precipitation of the luminatherefrom. The process described is therefore economical on accountof-the small amount of sodium compound which must be added in thefurnace. and even this is recovered in the residual liquor and used overagain. The other rea ent, to-wit, the lime or limestone, is very cheapand this remains in the "calcium silicate residue which is suitable forthe manufacture of cement.

The prqcessutilizes aluminous material which has heretofore beenconsidered too low grade for use in the furnace method of making sodiumaluminate, to-wit, red mud, or low grade aluminous materials such ascertain bauxites and natural clay, which are very widely distributed andof practically unlimited supply.

The process-covered by the claims hereinafter made is applicable to thetreatment of not only the particular aluminous ma terials' named herein,but also to the treatsuch solutions,

ment of any aluminous material by a furnacing process, in which asintered mass is produced for subsequent leachin \Vhat I claim is l. Theherein described method of obtaining alumina from aluminous materials ofhigh silica content, comprising mixing with the aluminous material analkaline reagent containing lime and sodium compound, the latter in theproportion, by weight. of about one molecular equivalent of sodiumcompound to one molecular equivalent of alumina in the aluminousmaterial; heating the mixture to produce a compound of lime and silica:leaching the product with residual sodium-aluminate solution to dissolvealumina as sodium aluminate, at a tempera-' ture below 212 F. and above190 F., approximately; heating the resulting liquor under pressure to atemperature above 212 F. to precipitate silica; and extracting aluminafrom the solution.

2. The herein described method of obtaining alumina from aluminousmaterials of high silica content, comprising mixing lime andsodiumcompound with the aluminous material, the soda in the proportion, byweight, of about one molecular equivalent ,of sodium compound to onemolecular equivalent of alumina in the material; heating the mixture toproduce a compound of lime and silica; leaching the product with asodium compound solution at a temperature between 180 F. and 212 F. todissolve alumina as sodium ahuninate; and extracting alumina from thesolution.

3. The herein described method of obtaining alumina from aluminousmaterials of high silica content, comprising heating the alumina andsilica with lime and sodium compound to produce a compound of lime andsilica; leaching the product at a temperature of about 200 F. todissolve alumina as sodium .aluminate; and extracting alumina from thesolution.

4. The herein described method of obtaining alumina from materialscontaining alumina, lime, sodium compound, and a high proportion ofsilica, which consists in heating the materials to incipient sintering,leaching the product at a temperature of about 200 F. to dissolvealuminaas sodium aluminate; and extracting alumina from the solution.

5. The herein described method of obtaining alumina from materialscontaining alumina, sodium compound, lime and a high proportion ofsilica, which consists in heating the materials to produce a compound oflime and si1ica,-leaching the product to dissolve alumina as sodiumaluminate, heating the solution under pressure to a temperatureexceeding 212 F. to precipitate silica, and extracting alumina from thesolution thus purified.

ture, but not to fusion,

6. The herein described method of obtaining alumina from materialscontaining alumina, lime, sodium compound, and a high proportion ofsilica, which consists in heating the material to produce a compound oflime and silica, leaching the product to dissolve alumina as sodiumaluminate, heating.

the solution under pressure to a temperature of from 280 F. to 300 toprecipitate silica,

and extracting alumina from the solution thus purified.

7. The herein described method of obtaining alumina from materialscontaining alumina, lime, sodium compound, and a high proportion ofsilica, the sodium compound being in the proportion, by weight, of aboutone molecular equivalent of sodium compound to one molecularequivalent'of alumina, which consists in heating the material,

producing a compound of silica and lime ual liquor resulting from theprecipitation of alumina from a sodium aluminate solution to dissolvealumina as sodium aluminate, and precipitating alumina from the solutionthus produced.

9. The herein described process which comprises heating to. a sinteringtemperaan intimate mixture comprising an' aluminous-siliceous material,a calcareous material and a sodium compound, the niblecular ratio oflime to silica being tween 2:1 and 0.6:1, and thereafter leachbetween2.8:1 and 12:1 and the molecular ratio of soda to alumina being be- 7ing the sintered mass with an alkaline solution containing a sodiumcompound, the molecular ratio of total potential NaOH in the sinteredproduct and leaching liquor to total potential A1 0 in the sinteredproduct and leaching liquor being between'about 4 1 and about 5 :1.

'10. In the leaching of sintered unfused alumina-silica-lime-sodamixtures, the step of heating to not above 200 F., such a sinteredmixture and an alkaline solution of a sodium compound.

11. In the leaching of sintered unfused alumina-silica-lime-sodamixtures, the step of leachin such sintered product with an alkaline soution of a sodium compound, the molecular ratio of total available NaOHin the sintered product and leaching liquor,

and the total available A1 0 in the sintered product and leachingliquor. being between about 4:1 and about5:l.

12. The herein described method of obtaining alumina from aluminousmaterials of high silica content, com-prising heating the alumina andsilica with lime and sodium compound to produce a compound of lime andsilica; leaching the product at a temperature not over about 200 F.,with an alkaline solution containing sodium comq pounds, to dissolvealumina as sodium aluminate; and extracting alumina from the solution. i

' 13. In the treatment of sodium aluminate liquors containing silica,the step of desilicitying the same by heating under pressure to atemperature substantially above the normal boiling point, but notsubstantially above 300- F., until a substantial part at least of thesilica is precipitated, and sepa-v rating the precipitated silica from'the' mother liquor. Y

In testimony whereof, I have hereunto set my hand.

RALPH S; SHERWIN. itnesses G. E. HoDosoN,

H. P. HEHNER.

